This invention relates to a process for producing a fiber composite material and to a fiber composite material containing fibers with a high hot strength, a pressing compound produced from fibers, a binder and, if appropriate, fillers and/or additives. More particularly, the fibers are based on carbon, silicon, boron, and/or nitrogen. The mass is then pressed in a press mold to form a green body.
A process of the generic type and a ceramic composite material of the generic type are described in German Patent Application 197 11 829.1, which is not a prior publication. The reinforcing fibers which are known from this document are fibers with a high hot strength which are present in the form of short fiber bundles. The fiber bundles are impregnated with a binder which is suitable for pyrolysis. For this purpose, the fiber bundles are dipped into the binder. The binder is then solidified. Consequently, the fiber bundles are held together and mechanically reinforced. The fiber bundles are mixed with further binders and fillers and the mixture is hot-pressed to form a CRP body or xe2x80x9cgreen bodyxe2x80x9d, which is then pyrolyzed in vacuo or under an inert gas to form a shaped body with a carbon matrix (C/C body). In the process, the fiber coating is also converted so that the fiber bundles are then coated with a layer of carbon. The shaped body is then infiltrated with molten silicon. The result is a C/SiC fiber composite material in which the fiber bundles are embedded in a matrix based on SiC. The short fiber bundles are embedded in the matrix in a randomly distributed form, with the individual filaments being substantially maintained. The carbon coating has reacted with the matrix material. As a result, the fiber bundles are protected from the aggressive attack from the molten silicon. This fiber composite ceramic exhibits very good tribological properties and, furthermore, is relatively inexpensive and easy to produce. It is suitable in particular for the production of brake discs and/or brake linings.
However, this material is unable to withstand particularly high mechanical loads, such as for example those which are generated by high vehicle masses or extreme speeds, since it is too brittle and insufficiently tolerant to damage to do so.
Various solutions have already been proposed in order to circumvent this problem. German Utility Model 296 10 498 describes a vehicle brake disc or vehicle clutch disc made from C-C/SiC composite material in which the disc has an SiC coating. Therefore, the outer region of the disc is made from ceramic material and provides very good frictional characteristics, while the core is a carbon body which, due to its pseudoductility, has high tolerance to damage. However, bodies that are coated in this way are complex and therefore expensive to produce. For this reason, they are only used for special applications, for example in motor racing.
European Patent Application EP 0 564 245 likewise describes a multilayer material which, however, has to be provided with a protective layer in order to prevent silicon from penetrating into relatively deep regions. This too is a highly complex and expensive process.
Therefore, the object of the invention is to provide a fiber composite material of the above type which offers an even higher strength and improved pseudoductility of the component. A further object of the invention is to provide a process for producing this material, making the material simple and inexpensive to produce and therefore suitable for series production.
The solution consists in a process for producing a fiber composite material containing fibers with a high hot strength, based on carbon, silicon, boron and/or nitrogen, which are reaction-bonded to a silicon-based matrix, a pressing compound being produced from fibers, binder and, if appropriate, filler and/or additives, which is then pressed in a press mold to form a green body, wherein various pressing compounds are produced, which contain fibers of different qualities and/or in different proportions, and the press mold is filled with the various pressing compounds in a plurality of successive steps and in a fiber composite material containing fibers made by the process. The fibers preferably have a layer of carbon and/or pyrolytic carbon.
The process according to the invention is distinguished by the fact that, to produce the green body, the press is successively filled with the various pressing compounds, the inner pressing compound comprising fibers of a core which is tolerant to damage, and the outermost pressing compound comprising fibers in a ceramicized frictional coating.
The material according to the invention is therefore a gradient material, the advantage of which lies in the extremely simple production process according to the invention.
According to the invention, during the production of the green body, the pressing compounds, during filling, are to be layered in the press mold in such a way that in the final component the frictional layer which has a high wear resistance and is largely ceramicized merges continually into a core which is tolerant to damage. In this way, the high wear resistance is combined with very good mechanical characteristics.
Therefore, if the mechanical loads on the component are extremely high, it is possible to further increase strength and extension characteristics, as can be demonstrated for example in the 3-point bending test. Under particularly high mechanical loads, such as for example those caused by high vehicle masses or extreme speeds, it is possible to adapt known processes for the low-cost production of fiber-reinforced composite ceramic in such a way that the material or the component offers a high strength and a very good resistance to wear on the outside, combined with a significantly increased pseudoductility on the inside.
The advantage of the process according to the invention is that there is no need to join layers with different properties using complex joining processes. In this case, the gradient is produced solely by the way in which the mold is filled. Due to the process used, the individual layers do not have any defined interlayers.
The filling heights required can be determined according to the particular application using tests on the compressibility of the various pressing compounds at constant pressure.
A highly ceramicized frictional layer on the component surface, for example the brake disc surface, is obtained by providing the fibers which have been processed in the pressing compound with coatings which make it possible for not only carbon-containing fillers and pyrolyzed binders but also carbon fibers to be partially converted by the molten silicon to form silicon carbide. This is achieved by applying known coatings in a suitably small thickness or using more reactive carbon-containing coatings.
As a result, the fibers which have been provided with a corresponding thin coating are relatively soft during processing to form the pressing compound. After mixing and pressing, they exhibit a high degree of interlacing. This means there are few, if any, spaces between them in which, for example, silicon can accumulate and therefore remain as unreacted residual silicon following the infiltration with liquid silicon. Furthermore, the fibers are reaction-bonded to the matrix. The result is a high proportion of ceramic fibers. The frictional layer formed therefore has a high strength with an excellent tolerance to damage and is characterized by a high resistance to wear. A brake disc produced using this process has, for example, a high coefficient of friction with suitably adapted linings.
A layer of pyrolytic carbon (PyC) is applied to at least some of the reinforcement fibers used. Only then is a simple dip coating in accordance with the known process carried out.
These preferred reinforcement fibers are therefore each individually coated with two additional layers. The bottom layer, which is applied direct to the fiber, is made from pyrolytic carbon. A dip-coating which is known per se comprising a pyrolyzable binder is applied to this layer. During the infiltration of the porous shaped body with liquid silicon, the layer of carbon resulting from the resin coating acts as a xe2x80x9csacrificial layerxe2x80x9d. The liquid silicon reacts with this outer layer to form silicon carbide. This forms a diffusion barrier to the liquid silicon, which therefore cannot penetrate further into the fiber. The deeper layer of pyrolytic carbon and the reinforcement fibers in the core are not attacked.
The fibers which have been treated in this way are distinguished by a particularly high strength. The additional layer of pyrolytic carbon also produces optimum bonding of the reinforcement fibers to the matrix. They have a crack-diverting action and can slide in the longitudinal direction, resulting in the good results of the strength and 3-point bending tests. fiber-pullout effects are possible.
By using these reinforcement fibers during the production of the fiber composite material according to the invention, even in small proportions of the total fiber volume, it is possible to significantly increase the strength and extension figures, as can be demonstrated, for example, using the 3-point bending test. They do not impair the other parameters.
By coating the PyC fibers with a resin solution, it is possible to use these fibers even for silicized materials.
The process for producing these reinforcement fibers is distinguished by the fact that carbon fibers are firstly coated with pyrolytic carbon. This term is understood to mean both pyrolyzed dip coatings, such as for example pitch, and layers deposited from the vapor phase. The fibers are then provided with pyrolyzable plastic material.
The coating with pyrolytic carbon may, on the one hand, be carried out by dip coating, for example by dipping into a pitch bath. This process is suitable in particular for long fibers. Alternatively, a CVD coating, for example using methane in a reactor, may be applied to the fibers. This process is eminently suitable for both long fibers and short fibers.
The use of pitch has the advantage that the pyrolytic carbon layer formed is crystalline carbon which reacts with liquid silicon significantly more slowly than a layer of amorphous carbon, as is formed, for example, when a phenolic resin is used. As a result, the diffusion barrier for the amorphous carbon is strengthened further.
Long fibers are preferably cut after the coating and before they are processed to form a green body.
It is possible to use treated individual fibers or fiber bundles. The fiber bundles preferably comprise approximately 1000 to 14,000 individual fibers, with mean diameters of approximately 5 to 10 xcexcm and a length of approximately 1 to 30 mm. In this way, it is also possible to use commercially available fiber bundles, allowing inexpensive production.
For the gradient material according to the invention, this means that the pressing compounds which have been layered successively into the press mold contain reinforcement fibers in which the quality of the fiber coating increases from the outside inwards. For example, in the core of a subsequent brake disc PyC-coated carbon fibers are used, so that the entire component is made tolerant to damage. Further filling is with pressing compounds which contain fibers of decreasing coating quality, until ultimately fibers with only a slight coatingxe2x80x94and in extreme cases even uncoated fibersxe2x80x94are used for the frictional layer. The outermost layer, which then serves as the actual frictional layer, may therefore comprise predominantly or even entirely silicon carbide, since the slightly coated or even uncoated fibers are predominantly or completely converted into silicon carbide during the liquid silicization.
Furthermore, it is possible to achieve the gradient in mechanical and tribological properties not only by using the fiber coating but also by varying both the fiber quality and the fiber length.
The use of short fibers has the further advantage that the filling and pressing operation also orients fibers perpendicular to the pressing plane, thus ensuring a continuous transition of the properties.
All customary reinforcement fibers can be used to produce the material according to the invention. Carbon fibers are preferred. However, other fibers with high hot strength, such as silicon carbide fibers or fibers based on Si/C/B/N, are suitable in principle. Furthermore, glass fibers or metal fibers, for example fibers based on titanium, are suitable. Aramid fibers are also eminently suitable.
These different variables, in combination, make it possible to produce a defined change in the materials"" properties over the thickness of the disc.